Arctic sea ice is a very dynamic environment which is currently suffering a rapid decline in extent and thickness. These changes will affect the distribution and magnitude of biogeochemical processes occurring in the ice matrix and the water column such as photosynthetic carbon fixation. Besides the phytoplankton in the surface waters of the Arctic Ocean, sea ice algae contribute to primary production, but our knowledge about their activity, especially in the central basins, is still limited. Of special interest for future predictions of the Arctic carbon cycle is the question which factors are limiting primary productivity in the ice, and if the sea ice retreat has positive or negative consequences for Arctic productivity.
This study provides recent data from the central Arctic, collected during August and September 2011 on board of the RV Polarstern (TransArc 2011). Net primary productivity (NPP) was measured using the 14C method in a range of ice types and features sampled along a transect from Atlantic to Pacific waters, including annual and multiyear ice flows and surface melt ponds. In addition, transparent exopolymers (TEP), particulate organic carbon (POC) and inorganic nutrients were determined.
The preliminary results show high spatial variability of NPP rates for the ice (7-24 mg C m-2 d-1) and the water column mixed layer depth integrated (1,8-333 mg C m-2 d-1) with remarkable differences between the Atlantic and the Pacific influenced waters. However, TEP concentrations in sea ice (254-1293 µg Xeq L-1) are significantly higher than in the water column. In addition, algal aggregates found in the melt ponds show very high activities and great concentrations of TEP. Regarding the nutrients, nitrate and phosphate concentrations are lower in the ice compared to the water column indicating a possible nutrient limitation for sea ice algae. Furthermore, the very low N:P ratio (~2 for the ice and ~8 for the water column) points to a general nitrogen limitation.
These results contribute to broaden our understanding of the biogeochemical and ecological processes taking place in the Central Arctic at the end of the ice melt season and
will help to estimate the contribution of sea ice to the carbon budget of the Arctic Ocean.